1. Field of the Invention
The present invention relates to a method for manufacturing a resistor, and more particularly, to a method for manufacturing a resistor having low end-resistance.
2. Description of the Prior Art
In a semiconductor process, semiconductor materials, such as polysilicon, are often positioned to function as resistors capable of providing high resistance. These resistors can be used in place of load transistors. When load transistors of a static random access memory (SRAM) is replaced by semiconductor resistors, the number of transistors in the SRAM can be reduced and thus saves cost and enhance the integration of the SRAM.
Conventionally used semiconductor resistors can be divided into polysilicon resistors and diffusion resistors. A polysilicon resistor is formed, for example, by forming a polysilicon film into which dopants are doped into through an insulating film on a semiconductor substrate. The resistance of the polysilicon resistor can be controlled by the dopant concentration in the polysilicon layer. A diffusion resistor is formed, for example, by doping dopants into a semiconductor substrate to form a diffusion layer, and then by using a thermal diffusion method to activate the dopants in the diffusion layer and adjust the resistance thereof. Normally, the resistor, no matter it is a polysilicon resistor or a diffusion resistor, has a sandwich-like structure that sandwiches a high resistance region between two low resistance ends. The low resistance ends are provided for forming interconnection contact plugs to connect the resistor with other wirings. The high resistance region is used to provide a high resistance to satisfy circuit designs or device demands.
With the development of the various electronic products, circuit designs applying load resistors become more and more complicated. For example, for the analog/digital mixed mode integrated circuits or the radio frequency integrated circuits, it is required that the load resistors have a high value of ohmic resistance and the value of the ohmic resistance must further be within tight limits. However, the conventional semiconductor resistor structure has high end-resistance in the vicinity of the junction between the high resistance region and the low resistance region. As a result, a linearity relationship between characteristic parameters, such as resistance, temperature coefficients and voltage coefficients, and structure parameters, such as a length and a width of the resistor structure, is affected by the high end-resistance of the resistor. In this case, it becomes more difficult to calculate an accurate value of the resistance and provide a stable value of high resistance, thus decreasing yields of the products.